1. Field of the Invention
The present invention relates to a dummy mask used in a lithography process for semiconductor devices, and more particularly to an exposure process and a dummy mask adapted to form a pattern on a wafer by diffracted light to form an ultra-fine pattern.
2. Description of the Prior Art
Generally, the pattern of a mask is transferred onto a wafer by use of a stepper. Where the mask pattern is very fine, light incident on the mask is increased in diffraction angle when it emerges from the mask, thereby causing the resolution to be degraded. As a result, the pattern may be badly transferred onto the wafer.
A conventional light exposure technique for transferring a pattern onto a wafer will now be described, in conjunction with FIGS. 1 and 2.
FIG. 1 is a schematic view explaining a light path in a stepper. This light path is established when light 1 is vertically incident on a pattern mask 4.
Since the mask 4 is provided with a fine pattern, light transmitted through the mask 4 is increased in diffraction angle. As a result, the light emerging from a projection lens 5 is badly focused onto a wafer 6. Moreover, a great degradation in resolution may occur when the wafer 6 is inaccurately positioned.
For solving this problem, a light exposure method has been proposed involving an inclined light incidence capable of obtaining an improved depth of focus. For accomplishing such an inclined light incidence, a modified illumination mechanism for feeding light at a certain angle to a stepper is used. However, the modified illumination mechanism is limited in dimension and shape because it should be installed in a light exposure device. Moreover, line width of a pattern formed on a wafer may vary depending on the shape of the modified illumination mechanism. As a result, uniformity of the pattern line width may be degraded.
On the other hand, FIG. 2 is a schematic view illustrating another conventional light exposure method providing an improvement over the above-mentioned method illustrated in FIG. 1. This method uses a dummy mask involving light diffraction, for exposing a wafer to light.
As vertically incident light 1 passes through the dummy mask 2 fabricated in accordance with the diffraction principle, it is diffracted. The diffracted light passes through a Fresnel region 7 and then enters a pattern mask 4 formed with a predetermined pattern. The light emerging from the pattern mask 4 passes through a projection lens 5 and then transfers the pattern onto a wafer 6. The light incident on the pattern mask 4 after passing through the Fresnel region 7 includes main light components having an important affect on the formation of a pattern, namely, a zero-order diffracted light component 8, -1st diffracted light component 8' and +1st diffracted light component 8".
The diffracted light components 20' and 20" derived from the zero-order diffracted light component 8 vertically entering the pattern mask 4 having the pattern to be transferred onto the wafer 6 is incident on the projection lens 5 along the same paths as those in the case shown in FIG. 1. As a result, the zero-order diffracted light component 8 exhibits a small depth of focus. On the other hand, the -1st and +1st diffracted light components 8' and 8" are inclinedly incident on the pattern mask 4, thereby exhibiting a large depth of focus. In other words, the diffracted light components 9' and 9" incident on the projection lens 5, which are derived respectively from the -1st and +1st diffracted light components 8' and 8", exhibit a larger depth of focus than that of the diffracted light components 20' and 20". As a result, a pattern exhibiting a superior resolution can be obtained.
In FIG. 2, the reference numeral 9 denotes a light component diffracted at an angle beyond the incidence range of the projection lens 5.
Although an increase in depth of focus is realized by the inclined incidence, the conventional method still involves the problem encountered in formation of ultra-fine patterns because the wafer is exposed to a considerable amount of light components vertically incident on the mask formed with the pattern to be transferred.